// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#ifndef DORIS_BE_SRC_OLAP_SKIPLIST_H
#define DORIS_BE_SRC_OLAP_SKIPLIST_H

// Thread safety
// -------------
//
// Writes require external synchronization, most likely a mutex.
// Reads require a guarantee that the SkipList will not be destroyed
// while the read is in progress.  Apart from that, reads progress
// without any internal locking or synchronization.
//
// Invariants:
//
// (1) Allocated nodes are never deleted until the SkipList is
// destroyed.  This is trivially guaranteed by the code since we
// never delete any skip list nodes.
//
// (2) The contents of a Node except for the next/prev pointers are
// immutable after the Node has been linked into the SkipList.
// Only Insert() modifies the list, and it is careful to initialize
// a node and use release-stores to publish the nodes in one or
// more lists.
//
// ... prev vs. next pointer ordering ...

#include <atomic>

#include "common/logging.h"
#include "gen_cpp/olap_file.pb.h"
#include "runtime/mem_pool.h"
#include "util/random.h"

namespace doris {

template<typename Key, class Comparator>
class SkipList {
private:
    struct Node;
    enum { kMaxHeight = 12 };

public:
    typedef Key key_type;
    // One Hint object is to show position info of one row.
    // It is used in the following scenarios:
    //   // 1. check for existence
    //   bool is_exist = skiplist->Find(key, &hint);
    //   // 2. Do something separately based on the value of is_exist
    //   if (is_exist) {
    //       do_something1 ();
    //   } else {
    //       do_something2 ();
    //       skiplist->InsertWithHint(key, is_exist, hint);
    //   }
    //
    // Note: The user should guarantee that there must not be any other insertion
    // between calling Find() and InsertWithHint().
    struct Hint {
        Node* curr;
        Node* prev[kMaxHeight];
    };

    // Create a new SkipList object that will use "cmp" for comparing keys,
    // and will allocate memory using "*mem_pool".
    // NOTE: Objects allocated in the mem_pool must remain allocated for
    // the lifetime of the skiplist object.
    explicit SkipList(Comparator cmp, MemPool* mem_pool, bool can_dup);

    // Insert key into the list.
    void Insert(const Key& key, bool* overwritten);
    // Use hint to insert a key. the hint is from previous Find()
    void InsertWithHint(const Key& key, bool is_exist, Hint* hint);

    // Returns true iff an entry that compares equal to key is in the list.
    bool Contains(const Key& key) const;
    // Like Contains(), but it will return the position info as a hint. We can use this
    // position info to insert directly using InsertWithHint().
    bool Find(const Key& key, Hint* hint) const;

    // Iteration over the contents of a skip list
    class Iterator {
    public:
        // Initialize an iterator over the specified list.
        // The returned iterator is not valid.
        explicit Iterator(const SkipList* list);

        // Returns true iff the iterator is positioned at a valid node.
        bool Valid() const;

        // Returns the key at the current position.
        // REQUIRES: Valid()
        const Key& key() const;

        // Advances to the next position.
        // REQUIRES: Valid()
        void Next();

        // Advances to the previous position.
        // REQUIRES: Valid()
        void Prev();

        // Advance to the first entry with a key >= target
        void Seek(const Key& target);

        // Position at the first entry in list.
        // Final state of iterator is Valid() iff list is not empty.
        void SeekToFirst();

        // Position at the last entry in list.
        // Final state of iterator is Valid() iff list is not empty.
        void SeekToLast();

    private:
        const SkipList* list_;
        Node* node_;
        // Intentionally copyable
    };

private:
    // Immutable after construction
    Comparator const compare_;
    // When value is true, means indicates that duplicate values are allowed.
    bool _can_dup;
    MemPool* const _mem_pool;    // MemPool used for allocations of nodes

    Node* const head_;

    // Modified only by Insert().  Read racily by readers, but stale
    // values are ok.
    std::atomic<int> max_height_;   // Height of the entire list

    inline int GetMaxHeight() const {
        return max_height_.load(std::memory_order_relaxed);
    }

    // Read/written only by Insert().
    Random rnd_;

    Node* NewNode(const Key& key, int height);
    int RandomHeight();
    bool Equal(const Key& a, const Key& b) const { return (compare_(a, b) == 0); }

    // Return true if key is greater than the data stored in "n"
    bool KeyIsAfterNode(const Key& key, Node* n) const;

    // Return the earliest node that comes at or after key.
    // Return NULL if there is no such node.
    //
    // If prev is non-NULL, fills prev[level] with pointer to previous
    // node at "level" for every level in [0..max_height_-1].
    Node* FindGreaterOrEqual(const Key& key, Node** prev) const;

    // Return the latest node with a key < key.
    // Return head_ if there is no such node.
    Node* FindLessThan(const Key& key) const;

    // Return the last node in the list.
    // Return head_ if list is empty.
    Node* FindLast() const;

    // No copying allowed
    SkipList(const SkipList&);
    void operator=(const SkipList&);
};

// Implementation details follow
template<typename Key, class Comparator>
struct SkipList<Key,Comparator>::Node {
    explicit Node(const Key& k) : key(k) { }

    Key const key;

    // Accessors/mutators for links.  Wrapped in methods so we can
    // add the appropriate barriers as necessary.
    Node* Next(int n) {
        DCHECK(n >= 0);
        // Use an 'acquire load' so that we observe a fully initialized
        // version of the returned Node.
        return (next_[n].load(std::memory_order_acquire));
    }
    void SetNext(int n, Node* x) {
        DCHECK(n >= 0);
        // Use a 'release store' so that anybody who reads through this
        // pointer observes a fully initialized version of the inserted node.
        next_[n].store(x, std::memory_order_release);
    }

    // No-barrier variants that can be safely used in a few locations.
    Node* NoBarrier_Next(int n) {
        DCHECK(n >= 0);
        return next_[n].load(std::memory_order_relaxed);
    }
    void NoBarrier_SetNext(int n, Node* x) {
        DCHECK(n >= 0);
        next_[n].store(x, std::memory_order_relaxed);
    }

    private:
    // Array of length equal to the node height.  next_[0] is lowest level link.
    std::atomic<Node*> next_[1];
};

template<typename Key, class Comparator>
typename SkipList<Key,Comparator>::Node*
SkipList<Key,Comparator>::NewNode(const Key& key, int height) {
    char* mem = (char*)_mem_pool->allocate(
            sizeof(Node) + sizeof(std::atomic<Node*>) * (height - 1));
    return new (mem) Node(key);
}

template<typename Key, class Comparator>
inline SkipList<Key,Comparator>::Iterator::Iterator(const SkipList* list) {
    list_ = list;
    node_ = NULL;
}

template<typename Key, class Comparator>
inline bool SkipList<Key,Comparator>::Iterator::Valid() const {
    return node_ != NULL;
}

template<typename Key, class Comparator>
inline const Key& SkipList<Key,Comparator>::Iterator::key() const {
    DCHECK(Valid());
    return node_->key;
}

template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::Next() {
    DCHECK(Valid());
    node_ = node_->Next(0);
}

template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::Prev() {
    // Instead of using explicit "prev" links, we just search for the
    // last node that falls before key.
    DCHECK(Valid());
    node_ = list_->FindLessThan(node_->key);
    if (node_ == list_->head_) {
        node_ = NULL;
    }
}

template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::Seek(const Key& target) {
    node_ = list_->FindGreaterOrEqual(target, NULL);
}

template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::SeekToFirst() {
    node_ = list_->head_->Next(0);
}

template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::SeekToLast() {
    node_ = list_->FindLast();
    if (node_ == list_->head_) {
        node_ = NULL;
    }
}

template<typename Key, class Comparator>
int SkipList<Key,Comparator>::RandomHeight() {
    // Increase height with probability 1 in kBranching
    static const unsigned int kBranching = 4;
    int height = 1;
    while (height < kMaxHeight && ((rnd_.Next() % kBranching) == 0)) {
        height++;
    }
    DCHECK(height > 0);
    DCHECK(height <= kMaxHeight);
    return height;
}

template<typename Key, class Comparator>
bool SkipList<Key,Comparator>::KeyIsAfterNode(const Key& key, Node* n) const {
    // NULL n is considered infinite
    return (n != NULL) && (compare_(n->key, key) < 0);
}

template<typename Key, class Comparator>
typename SkipList<Key,Comparator>::Node*
SkipList<Key,Comparator>::FindGreaterOrEqual(const Key& key, Node** prev) const {
    Node* x = head_;
    int level = GetMaxHeight() - 1;
    while (true) {
        Node* next = x->Next(level);
        if (KeyIsAfterNode(key, next)) {
            // Keep searching in this list
            x = next;
        } else {
            if (prev != NULL) prev[level] = x;
            if (level == 0) {
                return next;
            } else {
                // Switch to next list
                level--;
            }
        }
    }
}

template<typename Key, class Comparator>
typename SkipList<Key,Comparator>::Node*
SkipList<Key,Comparator>::FindLessThan(const Key& key) const {
    Node* x = head_;
    int level = GetMaxHeight() - 1;
    while (true) {
        DCHECK(x == head_ || compare_(x->key, key) < 0);
        Node* next = x->Next(level);
        if (next == NULL || compare_(next->key, key) >= 0) {
            if (level == 0) {
                return x;
            } else {
                // Switch to next list
                level--;
            }
        } else {
            x = next;
        }
    }
}

template<typename Key, class Comparator>
typename SkipList<Key,Comparator>::Node*
SkipList<Key,Comparator>::FindLast() const {
    Node* x = head_;
    int level = GetMaxHeight() - 1;
    while (true) {
        Node* next = x->Next(level);
        if (next == NULL) {
            if (level == 0) {
                return x;
            } else {
                // Switch to next list
                level--;
            }
        } else {
            x = next;
        }
    }
}

template<typename Key, class Comparator>
SkipList<Key,Comparator>::SkipList(Comparator cmp, MemPool* mem_pool, bool can_dup) :
    compare_(cmp),
    _can_dup(can_dup),
    _mem_pool(mem_pool),
    head_(NewNode(0 /* any key will do */, kMaxHeight)),
    max_height_(1),
    rnd_(0xdeadbeef) {
        for (int i = 0; i < kMaxHeight; i++) {
            head_->SetNext(i, NULL);
        }
    }

template<typename Key, class Comparator>
void SkipList<Key,Comparator>::Insert(const Key& key, bool* overwritten) {
    // TODO(opt): We can use a barrier-free variant of FindGreaterOrEqual()
    // here since Insert() is externally synchronized.
    Node* prev[kMaxHeight];
    Node* x = FindGreaterOrEqual(key, prev);

#ifndef BE_TEST
    // The key already exists and duplicate keys are not allowed, so we need to aggreage them
    if (!_can_dup && x != nullptr && Equal(key, x->key)) {
        *overwritten = true;
        return;
    }
#endif

    *overwritten = false;
    // Our data structure does not allow duplicate insertion
    int height = RandomHeight();
    if (height > GetMaxHeight()) {
        for (int i = GetMaxHeight(); i < height; i++) {
            prev[i] = head_;
        }
        //fprintf(stderr, "Change height from %d to %d\n", max_height_, height);

        // It is ok to mutate max_height_ without any synchronization
        // with concurrent readers.  A concurrent reader that observes
        // the new value of max_height_ will see either the old value of
        // new level pointers from head_ (NULL), or a new value set in
        // the loop below.  In the former case the reader will
        // immediately drop to the next level since NULL sorts after all
        // keys.  In the latter case the reader will use the new node.
        max_height_.store(height, std::memory_order_relaxed);
    }

    x = NewNode(key, height);
    for (int i = 0; i < height; i++) {
        // NoBarrier_SetNext() suffices since we will add a barrier when
        // we publish a pointer to "x" in prev[i].
        x->NoBarrier_SetNext(i, prev[i]->NoBarrier_Next(i));
        prev[i]->SetNext(i, x);
    }
}

// NOTE: Already be checked, the row is exist.
template<typename Key, class Comparator>
void SkipList<Key,Comparator>::InsertWithHint(const Key& key, bool is_exist, Hint* hint) {
    Node* x = hint->curr;
    DCHECK(!is_exist || x) << "curr pointer must not be null if row exists";

#ifndef BE_TEST
    // The key already exists and duplicate keys are not allowed, so we need to aggreage them
    if (!_can_dup && is_exist) {
        return;
    }
#endif

    Node** prev = hint->prev;
    // Our data structure does not allow duplicate insertion
    int height = RandomHeight();
    if (height > GetMaxHeight()) {
        for (int i = GetMaxHeight(); i < height; i++) {
            prev[i] = head_;
        }
        //fprintf(stderr, "Change height from %d to %d\n", max_height_, height);

        // It is ok to mutate max_height_ without any synchronization
        // with concurrent readers.  A concurrent reader that observes
        // the new value of max_height_ will see either the old value of
        // new level pointers from head_ (NULL), or a new value set in
        // the loop below.  In the former case the reader will
        // immediately drop to the next level since NULL sorts after all
        // keys.  In the latter case the reader will use the new node.
        max_height_.store(height, std::memory_order_relaxed);
    }

    x = NewNode(key, height);
    for (int i = 0; i < height; i++) {
        // NoBarrier_SetNext() suffices since we will add a barrier when
        // we publish a pointer to "x" in prev[i].
        x->NoBarrier_SetNext(i, prev[i]->NoBarrier_Next(i));
        prev[i]->SetNext(i, x);
    }
}

template<typename Key, class Comparator>
bool SkipList<Key,Comparator>::Contains(const Key& key) const {
    Node* x = FindGreaterOrEqual(key, NULL);
    if (x != NULL && Equal(key, x->key)) {
        return true;
    } else {
        return false;
    }
}

template<typename Key, class Comparator>
bool SkipList<Key,Comparator>::Find(const Key& key, Hint* hint) const {
    Node* x = FindGreaterOrEqual(key, hint->prev);
    hint->curr = x;
    if (x != NULL && Equal(key, x->key)) {
        return true;
    } else {
        return false;
    }
}

}  // namespace doris

#endif  // DORIS_BE_SRC_OLAP_SKIPLIST_H
